Math · Stage 1
Reason that gaps between items in a row are one fewer than the items (off-by-one)
Count the posts, then the spaces between them — a straight row always has one fewer gap than items.
✦ Math📘 Stage 1 · S1⏱ ~9 min🧒 Ages 5–6★ Free to start
I can reason that the number of gaps between items in a row is one fewer than the items (gaps = items − 1).
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Kavi Builds a Fence 🪵
Kavi lines up fence posts in a straight row and counts them: 1, 2, 3, 4, 5 — five posts! Then Kavi counts the spaces between the posts for the rails: only 4 spaces! "The gaps are one fewer than the posts," laughs Kavi. In a straight row, the spaces between the things always come to one less than the things themselves.
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What your child will learn
1Count the posts, then count the gapsHere are 5 fence posts in a row. Count them: 1, 2, 3, 4, 5. Now count the spaces between neighbouring posts — point between 1&2, 2&3, 3&4, 4&5.
2The number line shows it tooA number line is a row of marks. Count the marks from 0 to 5 — then count the hops between them. Tap Hop and watch each hop jump the space between two marks.
3The rule as a bondFlip it into a number bond: the gaps plus 1 make the posts. For 5 posts: 4 gaps + 1 = 5 posts. So gaps = posts − 1.
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A peek inside
Kavi puts 5 fence posts in a straight row. How many gaps (spaces for rails) are between them?
count posts: 1 2 3 4 5 · gaps sit between pairs: (1-2)(2-3)(3-4)(4-5) = 4 · gaps = posts − 1 = 5 − 1
Try this at home: There are 6 fence posts in a straight row. How many gaps are between them?
Ready to play and learn?
Start the free interactive lesson, or print the handout for screen-free play.
Part of Math · Stage 1 · S1 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.